I define a 2-D model. Open 'display' and 'grid' and I see many WALLs under 'Surfaces' column. I chose some walls, howere, they can not be seen in the figure. Why? where are they from? Thank you.

I guess you have fluid and solid parts in your calculation. if the interface between the solid and liquid is not defined before, fluent creats walls/shadow-walls. If you display nothing else but these walls you should see them. hope that helps Gernot